In a sputtering deposition process ions are usually created by collisions between gas atoms and electrons in a glow discharge. The ions are accelerated into the target cathode by an electric field causing atoms of the target material to be ejected from the cathode surface. A substrate is placed in a suitable location so that it intercepts a portion of the ejected atoms. Thus, a coating of target material is deposited on the surface of the substrate.
Sputter coating is a widely used technique for depositing a thin film of material on a substrate. Sputtering is the physical ejection of material from a target as a result of gas ion bombardment of the target. In one form of this technique, known as DC sputtering, positive ions from a plasma discharge formed between an anode and a target cathode are attracted to and strike the target cathode, dislodging atoms from the target surface of the cathode thereby providing sputtering atoms. Some of the dislodged atoms impinge on the surface of the substrate and form a coating. In reactive sputtering a gaseous species is also present at the substrate surface and reacts with, and in some embodiments combines with, the atoms from the target surface to form the desired coating material.
In operation, when the sputter gas, e.g. argon is admitted into a coating chamber, the DC voltage applied between the target cathode and the anode ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged target. The ions strike the target with a substantial energy and cause target atoms or atomic clusters to be sputtered from the target. Some of the target particles strike and deposit on the wafer or substrate material to be coated, thereby forming a film.
In an endeavor to attain increased deposition rates and lower operating pressures, magnetically enhanced targets have been used. In a planar magnetron, the cathode includes an array of permanent magnets arranged in a closed loop and mounted in a fixed position in relation to the flat target plate. Thus, the magnetic field causes the electrons to travel in a closed loop, commonly referred to as a “race track”, which establishes the path or region along which sputtering or erosion of the target material takes place. In a magnetron cathode, a magnetic field confines the glow discharge plasma and increases the path length of the electrons moving under the influence of the electric field. This results in an increase in the gas atom-electron collision probability thereby leading to a much higher sputtering rate than that obtained without the use of magnetic confinement. Furthermore, the sputtering process can be accomplished at a much lower gas pressure.
A limitation to the utility of planar and cylindrical magnetrons in either reactive or non-reactive sputtering is that films deposited by sputtering have not achieved the degree of uniformity or repeatability required for many high precision applications.
It is desired to create a magnetron sputtering system that increases production and product uniformity across an individual substrate, from device to device and from run to run. Device geometry, particularly the relationship between the cathode and the objects to be coated, has a significant effect on the rate of deposition and the area coated, as well as product quality and consistency. Variation in layer thickness across a substrate is referred to as runoff. The runoff can be predicted through modeling the device geometry.
In many coating apparatuses masking is used to reduce the coating rate variation to acceptable levels. But over time the masks typically accumulate large amounts of coating material. Once the material reaches a critical thickness it may flake off and contribute to particles that compromise the coating quality. Also trimming and maintaining such masks are elaborate processes. It is desired to provide a device that does not use a mask.
The device geometry of rotating a single substrate around its own axis with an offset cathode where the vertical and lateral distance of the cathode to the substrate is about the same is well known. This geometry leads to low runoff without the use of a mask. However, much coating material is wasted in this configuration. References to this can be found, for example, in H. A. Macleod: Thin-Film Optical Filters (Institute of Physics Publishing, Dirac House, Temple Back, Bristol BS1 6BE, UK, 2001) and in P. Baumeister: Optical Coating Technology (SPIE, Bellingham, Wash.), 2004.
Alternatively, supporting multiple substrates on a multiple axis planetary drive system is also well known. This configuration is also discussed in the MacLeod and Baumeister references.
Positioning the central axis of a planetary drive system on axis with the center point of the cathode target, is demonstrated in Scobey U.S. Pat. No. 5,851,365 and also Baumeister chapter (9-37), which states, “it is advisable to position the source not far from the center of rotation to avoid large vapor impingement angles.” Bergfelt U.S. Pat. No. 4,222,345 also discloses an on axis geometry to minimize excursion per revolution but this device requires a special mask or large runoff results. The cathode target in Bergfelt is not specified. It appears to be a point source.
The device disclosed by Scobey U.S. Pat. No. 5,851,365 is a relatively slow, large batch size machine, which processes 15 inch substrates in the planetary drive. The size of this device makes it difficult to maintain the drive system at good tolerances, causing variation in coating quality and runoff. The attempt to increase production by increasing batch size is further limited by increased defect losses and reduced yield.
It is an object of this invention to provide a magnetron sputter coating device with a geometry that provides rapid coating while maintaining high coating quality and minimizing material waste.
It is an object of this invention to provide a magnetron sputter coating device and method of operation that further increases production by decreasing cycle time.
It is a further object of this invention to provide a sputter coating device that produces high quality coatings without the use of a mask.